1. Field of the Invention
This invention relates to an apparatus for producing a crucible of vitreous silica (silica glass) for use in the pulling of a silicon single crystal, and more particularly to an apparatus for producing a silica crucible suitable for forming a sintered layer of a semi-molten state on an outer wall portion of the crucible during the arc heating.
2. Description of the Related Art
In the pulling of a silicon single crystal is used a silica crucible placing a silicon melt therein. In the silica crucible, an inner wall portion (inner surface layer) is in contact with the silicon melt and is constituted with a silica glass layer substantially containing no air bubble, while an outer wall portion (outer surface layer) is constituted with a sintered layer containing many air bubbles for dispersing heat of external radiation to uniformly transfer to an inside of a mold.
As a method of producing the silica crucible is conventionally known a rotating mold method. This production method is a method wherein silica powder deposited onto an inner face of a rotating mold is vitrified by heating from a spacing side of the mold to produce a crucible. In this case, an inner wall portion made from a silica glass layer substantially containing no air bubble is formed by sucking air in the deposited layer of the silica powder from the side of the mold under a reduced pressure during the heat-melting to conduct vacuuming for removing air bubbles in the deposited layer (see JP-A-S56-149333 and JP-A-H01-148782, for example).
Recently, as the size of the crucible becomes bigger, the crucible temperature during the pulling tends to be higher. As the crucible temperature rises, the viscosity of the glass decreases and there is a fear of deforming the crucible in use. As a countermeasure, there is known a way that a glass crystallization accelerator is applied onto or included in the surface of the crucible to crystallize the glass forming the crucible under a high temperature to thereby enhance the strength of the crucible.
For example, JP-A-H08-2932 discloses that a crystallization accelerator (alkali metal, alkaline earth metal or the like) is applied onto a surface of an inner wall portion of the silica crucible or the crystallization accelerator is included in the inner wall portion so as to crystallize the inner wall portion of the crucible promptly using the crystallization accelerator as a nucleus. Also, JP-A-H11-199370 discloses a vitreous silica layer obtained by forming a layer containing a crystallization accelerator on a lower side of an inner wall portion made from a silica glass (side an outer wall portion) so as to prevent the crystallization accelerator from contacting with a silicon melt. Further, JP-A-2003-95678 discloses that a crystallization accelerator is attached on an outer wall portion as well as an inner wall portion of a crucible to enhance the dimensional stability of the inner wall portion of the crucible.
However, when the crystallization accelerator is applied on the surface of the inner wall portion of the crucible, there is a fear that the crystallization accelerator is incorporated into the silicon melt by contacting therewith to increase the impurity concentration in a silicon single crystal. On the other hand, when the crystallization accelerator is applied on the surface of the outer wall portion of the crucible, since the crystallization accelerator is at a state of contacting with a carbon container mounted with the crucible during the pulling, carbon is reacted with the crystallization accelerator under a higher temperature to generate a gas, which may deteriorate the quality of the silicon single crystal. Moreover, when the crystallization accelerator is included into the glass of the crucible, it is required to take means for incorporating the crystallization accelerator into silica or quartz powder in the production of the crucible, which makes the production of the crucible complicated.